There has been a controversy about the self-aggregation of 5,10,15,20-tetrakis[4-(N-methyl)pyridinium]porphyrin (TMPyP(4)) in water. In order to make clear whether TMPyP(4) forms a dimer in water, 1H NMR and UV−vis spectroscopic studies of 5-phenyl-10,15,20-tris[4-(N-methyl)pyridinium]porphyrin (TriMPyP), 5,10-diphenyl-15,20-bis[4-(N-methyl)pyridinium]porphyrin (5,10-DiMPyP), and 5,15-diphenyl-10,20-bis[4-(N-methyl)pyridinium]porphyrin (5,15-DiMPyP) have been carried out. The 1H NMR spectra indicate the formation of the self-aggregates of these phenylpyridiniumporphyrins in D2O when the porphyrin concentration is 1 × 10-3 M. Comparison of the 1H NMR spectra of TMPyP(4) with those of TriMPyP and DiMPyPs clearly exhibits that TMPyP(4) does not form its dimer in D2O. Broadening of the signal due to the β-pyrrole protons of the cationic porphyrin is ascribed to the tautomerism of the inner N-D deuterons with the rate slower than that of the N-H protons. At lower concentrations (∼10-5 M), both DiMPyP's form dimers in water in the presence of KNO3, while TMPyP(4) and TriMPyP exist as monomers. The association constants for dimerization in water at 25 °C have been determined to be 7.36 × 105 M-1 for 5,10-DiMPyP in the presence of 0.05 M KNO3 and 1.12 × 106 M-1 for 5,15-DiMPyP in the presence of 0.01 M KNO3. Large and negative ΔH and ΔS values for dimerization of these DiMPyP's suggest the London's dispersion force as the main binding force.

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Thermodynamic parameters for complexation of polyvalent cyclodextrin (CD) cation and anion with oppositely charged guests have been determined in D2O containing 0.02 M NaCl by means of 1H-NMR spectroscopy. Protonated heptakis(6-amino-6-deoxy)-beta-CD (per-NH3+-beta-CD) forms stable inclusion complexes with monovalent guest anions. The enthalpy (deltaH) and entropy changes (deltaS) for complexation of per-NH3+-beta-CD with p-methylbenzoate anion (p-CH3-Ph-CO2-) are 3.8 +/- 0.7 kJ mol(-1) and 88.6 +/- 2.2 J mol(-1) K(-1), respectively. The deltaH and deltaS values for the native beta-CD-p-CH3-Ph-CO2- system are -8.6 +/- 0.1 kJ mol(-1) and 15.3 +/- 0.7 J mol(-1) K(-1), respectively. The thermodynamic parameters clearly indicate that dehydration from both the host and guest ions accounts for the entropic gain in inclusion process of p-CH3-Ph-CO2- into the per-NH3+-beta-CD cavity. The fact that the neutral guests such as 2,6-dihydroxynaphthalene and p-methylbenzyl alcohol hardly form the complexes with per-NH3+-beta-CD exhibits that van der Waals and/or hydrophobic interactions do not cause the complexation of the polyvalent CD cation with the monovalent anion. The acetate anion is not included into the per-NH3+-beta-CD cavity, while the butanoate and hexanoate anions form the inclusion complexes. The complexation of the alkanoate anions is entropically dominated. Judging from these results, it may be concluded that Coulomb interactions cooperated with inclusion are required for realizing the large entropic gain due to extended dehydration. Entropically favorable complexation was also observed for the anionic CD-cationic guest system. The present study might present a general mechanism for ion pairing in water.

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